In consideration of environmental problems in the automotive industry, in particular, weight reduction of automotive bodies has progressed in recent years, which requires metal sheet materials used for automotive bodies to have smaller thickness while having high strength and rigidity. In view of these requirements, high-strength steel sheets have gradually been adopted as a metal material.
When, however, a high-strength steel sheet is used, a phenomenon, specifically, a large amount of springback occurs in the steel sheet during forming. If a large amount of springback occurs in a sheet, the sheet is described as having a defective shape and fails to be assembled by joining or by other methods. Thus, a springback reduction method is an extremely important technology and various different methods have been developed.
Patent Literature 1 is taken as an example of a springback reduction method. Patent Literature 1 discloses a die for press forming with which a bending step is performed without deep drawing and a vertical-wall-portion compressing step is performed immediately after the bending step is finished. In the bending step, a metal material is processed so as to have a hat-shaped cross section. In the vertical-wall-portion compressing step, a compressive force is applied to a vertical wall portion of the metal sheet material. The bending step and the vertical-wall-portion compressing step prevent residual stress from occurring during press forming, whereby springback is reduced.